Conventionally, a bicycle control cable is interposed between, for example, a derailleur and a control device therefor which includes an operating lever for operating the derailleur. The operating lever is operated to actuate the derailleur through a control wire so that a chain guide constituting an element of the derailleur shifts a driving chain from one sprocket to another of a multistage sprocket assembly.
The aforesaid control device, such as a lever shifting device, is provided with a positioning mechanism for positioning the operating lever in the operation position corresponding to the speed-change stage. The positioning mechanism comprises a plurality of engaging portions corresponding to the number of speed-change stages, a retainer in the form of, for example, a ball engageable selectively with individual engaging portions, and a spring for biasing the retainer toward the engaging portions. The engagement of the retainer with one engaging portion positions the lever corresponding to the selected speed-change stage so that the lever is maintained in this position against a return spring. Hence, the lever can keep the chain guide of the derailleur in position corresponding to a desired selected particular sprocket of the multistage sprocket assembly. However, when the control wire, after numerous repetitions of speed change operation, becomes extended in length relative to an outer sheath of the control cable, the stroke of the operating lever remains constant corresponding to an operation interval set by the positioning mechanism, whereby the extended control wire conversely causes an insufficient movement of the chain guide.
The lever shifting device provided with the positioning mechanism holds the operating lever in position corresponding to the selected speed change stage, so that the operating lever cannot be rotated an additional amount even when the control cable has become extended, whereby the movement of the chain guide falls short of its necessary movement to yield an effective and optimal speed change operation. As a result, the movement of the chain guide becomes improper with respect to each desired sprocket, whereby the chain guide stops out of the stable region thereof.
Hence, the chain shifted by the chain guide is slanted with respect to the sprocket to engage with the chain so as to make contact with the chain guide or the tooth crest of the sprocket, resulting in the production of noises.
To solve the above problem, the derailleur is provided with an adjusting bolt which is rotated to adjust the position where the outer sheath is held to thereby change the length thereof relative to the control wire, thereby correcting the chain guide to be kept in the stable region.
It is noted that the aforesaid stable region is defined as a region where the chain is shifted by the chain guide to a desired sprocket without striking the desired sprocket or the chain guide, which has a fixed width with respect to the sprocket.
However, the above solution is inadequate because, in a case where the adjusting bolt is rotated to adjust the control cable when the chain strikes the sprocket or the chain guide, it is unknown to what extent the chain guide has moved out of the stable region and also there is no standard for adjusting the length of the control cable. Hence, the adjustment may be insufficient to result in an increase in the frequency of adjustment, thereby creating a problem in that the sprocket and chain guide will cause frequent wearing thereof.
In addition, a control cable for a bicycle brake has the same problem as described above.